In U.S. Pat. No. 3,873,804 a welder's helmet is disclosed utilizing a protective welding lens assembly comprising a layer of nematic liquid crystal material sandwiched between opposing parallel plates coated with transparent conductive films. Polarizers are disposed on opposite sides of the plates such that by applying an electrical field across the transparent conductive films, the opacity of the liquid crystal light shutter can be changed.
In one embodiment of the invention shown in the aforesaid U.S. Pat. No. 3,873,804, light energy is sensed from the welding arc by means of a phototransistor or the like in order to cause the liquid crystal light shutter of the lens assembly to change from a light-transmitting condition, wherein the welder can view the work as illuminated by ambient light, to a substantially opaque condition when the arc is struck, the opaque condition being such as to transmit less than 1% of the visible light.
While a phototransistor of the type described above is a convenient means of detecting the existence of a welding arc, such an arc is a small portion of the field of view of the phototransistor comprising the area viewed by the welding helmet user. In this respect, visible light energy from the welding arc, at a distance of a few feet, is generally one and not more than ten times the magnitude of the ambient illumination in a welding shop and often less than one. As a result, detecting the visible light from the welding arc is not a satisfactory means of triggering the light shutter into a substantially opaque condition.
The liquid crystal light shutter of a welding helmet can be triggered into a substantially opaque condition by infrared wave energy prevalent in the welding arc but not present in fluorescent illumination, the most common illumination for welding shops. However, this also presents problems for the reason that when the welding helmet is close to the arc, and after the arc is extinguished, a phototransistor responsive to infrared radiation will receive a detectable level of such infrared radiation from the still-hot welding bead. Consequently, the liquid crystal light shutter, which the infrared detector system controls, will not open for periods of several seconds and will remain substantially opaque after the arc is extinguished so that the welder cannot view the work under ambient illumination in a welding shop.